So how are the integral proteins protected from proteolytic
degradation in the vacuole? A clue to answer this question
may arise from the observation by Pedrazzini et al. (2016)
that long luminal polypeptide domains are not very frequent in the known tonoplast proteome. Evolutionary constraints
may therefore have favoured the formation of another glycan-independent
mechanism to protect proteins from harmful
intracellular activities in plants. From the perspective of a
plant glycobiologist this finding is not fruitful, but sometimes
we just have to accept that proteins do well in the absence of
sugars